﻿4 
  EDWIN 
  S. 
  GOODUICH. 
  

  

  have 
  contractile 
  muscular 
  walls 
  (fig. 
  4). 
  When 
  tlie 
  filament 
  is 
  

   fully 
  expanded 
  the 
  vessels 
  are 
  swollen 
  with 
  blood, 
  and 
  in 
  

   optical 
  section 
  appear 
  to 
  fill 
  almost 
  the 
  entire 
  cavity 
  of 
  the 
  

   gill, 
  being 
  separated 
  from 
  each 
  other 
  by 
  a 
  narrow 
  longitudinal 
  

   septum 
  (figs. 
  2 
  and 
  3, 
  5-). 
  At 
  other 
  times 
  the 
  vcsrsels 
  may 
  

   become 
  emptied 
  ; 
  their 
  walls 
  then 
  contract, 
  so 
  that 
  the 
  lumen 
  

   is 
  almost 
  or 
  entirely 
  obliterated. 
  This 
  is 
  the 
  case, 
  as 
  a 
  rule, 
  

   in 
  preserved 
  specimens 
  ; 
  and 
  snch 
  gill 
  filaments, 
  when 
  cut 
  in 
  

   cross-sectiun, 
  present 
  the 
  appearance 
  described 
  by 
  Yejdovsky 
  

   and 
  Rietsch,of 
  possessing 
  two 
  large 
  coelomic 
  cavities 
  sepai-ated 
  

   by 
  a 
  strong 
  longitudinal 
  septum. 
  It 
  will 
  be 
  understood, 
  

   however, 
  that 
  this 
  apparent 
  septum 
  is 
  formed 
  by 
  the 
  collapsed 
  

   walls 
  of 
  the 
  blood-vessels, 
  and 
  is 
  therefore 
  at 
  right 
  angles 
  to 
  

   the 
  true 
  septum 
  separating 
  the 
  vessels 
  in 
  a 
  distended 
  

   condition. 
  Fig. 
  4 
  shows 
  these 
  vessels 
  in 
  a 
  half-contracted 
  

   state. 
  As 
  for 
  the 
  lining 
  of 
  the 
  cavities 
  on 
  either 
  side, 
  it 
  

   appears 
  to 
  be 
  continuous 
  with 
  the 
  coelomic 
  epithelium 
  of 
  the 
  

   body-cavity, 
  although 
  the 
  cells 
  are 
  often 
  vevy 
  irregularly 
  

   disposed. 
  

  

  Now, 
  when 
  we 
  come 
  to 
  examine 
  the 
  blood-vessels 
  supplied 
  to 
  

   the 
  base 
  of 
  the 
  gills, 
  Ave 
  find 
  that 
  there 
  are 
  not 
  two, 
  but 
  

   three 
  running 
  to 
  each 
  filament. 
  The 
  main 
  dorsal 
  vessel 
  

   situated 
  on 
  the 
  intestine 
  (fig. 
  1, 
  dr) 
  gives 
  off: 
  behind 
  a 
  short 
  

   thick 
  branch, 
  which 
  soon 
  divides 
  into 
  two 
  limbs. 
  From 
  the 
  

   right 
  and 
  the 
  left 
  limb 
  come 
  off 
  in 
  regular 
  alternate 
  succes- 
  

   sion 
  two 
  rows 
  of 
  oifshoots, 
  the 
  dorsal 
  branchial 
  vessels 
  (figs. 
  

   1, 
  7, 
  8, 
  and 
  14). 
  These 
  generally 
  expand 
  into 
  two 
  marked 
  

   swellings, 
  then 
  narrow 
  down 
  to 
  straight 
  vessels 
  running 
  to 
  

   the 
  branchial 
  perforated 
  plates. 
  It 
  is 
  this 
  region 
  of 
  the 
  

   branchial 
  "artery" 
  which 
  is 
  said 
  to 
  be 
  supported 
  by 
  an 
  

   " 
  axis," 
  and 
  it 
  is 
  just 
  this 
  region 
  Avhich 
  has 
  been 
  strangely 
  

   misunderstood 
  by 
  previous 
  observers. 
  

  

  For 
  the 
  sake 
  of 
  clearness 
  in 
  description 
  we 
  may 
  subdivide 
  

   the 
  dorsal 
  branchial 
  vessel 
  into 
  three 
  regions 
  : 
  the 
  first 
  is 
  

   generally 
  marked 
  off 
  as 
  a 
  conspicuous 
  swelling, 
  it 
  is 
  the 
  

   portion 
  nearest 
  the 
  dorsal 
  vessel; 
  the 
  third 
  is 
  the 
  much 
  longer 
  

   and 
  narrower 
  region 
  supported 
  by 
  the 
  " 
  axis," 
  and 
  reaching 
  

  

  